Electric vehicles and energy storage power stations and the like generally require power batteries with a high capacity as power supplies. These power batteries should have, in addition to a high capacity, good security and a long cycle life to meet the standards for employment and people's needs.
In the prior art, when a power battery is overcharged, decomposition of the electrolyte in the power battery will generate excessive heat inside the power battery, thus causing the battery to fire; or it will cause the pressure inside power battery to increase, thus resulting in explosion of the battery. Therefore, before the power battery is out of control, it needs to cut off a connecting part of a positive tab and a positive terminal with the aid of an external short circuit structure to prevent the power battery from being continually charged. Under normal circumstances, in a power battery cap structure, there is a resistance provided between a positive terminal and a cap sheet so that the positive terminal is electrically connected with the cap sheet through the resistance, while a short circuit part is provided near a negative terminal, and the short circuit part acts upwards when the pressure inside the power battery increases so that a loop is formed between the negative and positive of the power battery, and a great current is generated in the loop to melt off the connecting part so as to cut off the main loop. In order to satisfy the current for melting off the connecting part, it is required that the resistance value of the resistance between the positive and cap sheet of the power battery be small. However, in the case of nailing abuse of a power battery, if the resistance value of the resistance between the positive and cap sheet of the power battery is small, the current of the external short circuit will be too large, and the nailing situation will spark, which will ignite the electrolyte and cause the battery to catch fire. Therefore, in the case of nailing abuse, the resistance value of the resistance between the positive and cap sheet of the power battery is required to be very large.
Based on the above mentioned, the requirements for the resistance value between the positive and cap sheet of a power battery in the cases such as overcharge or nailing abuse are contradictory in the prior art, and thus the protection for both overcharge and nailing abuse of the power battery cannot be satisfied simultaneously regardless of either the value of resistance of the cap structure of the power battery is large or small.